Ask HN: Free online courses for gifted maths students

A friend overseas has a 14-year-old who is very strong at math, but the family doesn’t have much money.

Are there good free online resources for such kids to help them explore and find their preferred directions, from reputable institutions?

A couple I’ve found:

1. https://science.mit.edu/diversity-and-inclusion/outreach-programs/#K-12

2. https://mity.org/online-programs/

Any others?